REMOVING RESONANCE EFFECTS FROM QUANTUM-MECHANICAL VIBRATIONAL PARTITION-FUNCTIONS OBTAINED FROM PERTURBATION-THEORY

This paper compares different methods for removing resonance effects f rom second-order perturbation theory calculations of vibrational energ ies in a variety of systems containing from two to six modes. Both the recently proposed method of Kuhler et al. and the standard approach o f Nielsen yield stable energy levels even very close to resonance, wit h the latter giving smaller average errors in such cases. In addition, the method of Kuhler et al. is observed to affect the ground-state en ergy, unlike the standard approach. This generally worsens the accurac y of the vibrational partition function at room temperature, especiall y for systems close to resonance. (C) 1998 American Institute of Physi cs.